I had a requirement from a client to remove the hover from the header navigation on the customer portal.
Here is how we do it, if we inspect, we can see that this is handled by a style element “.navbar-inverse .navbar-nav > li > a:hover“
Now if we removed the background colour here is how it looks.
Now let’s make these changes in the code, edit your portal and open the portal in the Power Apps gallery and go to themes.
Search for the style element and comment or remove the background colour.
Hope this helps!